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A chain reaction is a self-sustaining sequence of nuclear fission events. Option B is correct: In nuclear fission, when U-235 absorbs a neutron and splits, it releases 2-3 new neutrons. These new neutrons can each cause fission in other U-235 nuclei, releasing more neutrons, which cause more fissions, and so on. This self-propagating sequence is a chain reaction. If each fission releases exactly 1 neutron that causes another fission, it is a controlled chain reaction (used in reactors). If more than 1 neutron causes further fission, it becomes an uncontrolled chain reaction (atomic bomb). Option A is incorrect: Chain reactions are self-sustaining; they do not require external energy once started. The initial fission provides energy for subsequent reactions. Option C is incorrect: While a controlled chain reaction can be slow, the definition does not restrict it to being slow. An uncontrolled chain reaction is extremely fast and violent. Option D is incorrect: Chain reactions occur in nuclear reactors and bombs on Earth, not only in stars. Nuclear fusion occurs in stars.
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